The invention relates to a method of agglomeration for the recovery and size upgrading of small particles, for example from waste streams from industrial and mining activities.
The term small particles is used in this specification with reference to particles which have an average particle size of 3 mm or less.
In various industrial and mining activities products and by-products are produced either in the form of small particles or as dust. These products are often in the form of waste or are dangerous, difficult and costly to handle and transport.
For example carbonaceous particles, when mixed with air can lead to explosion and as a fuel the carbonaceous particles have the tendency to suppress combustion by oxygen occlusion. When metallic ores are added to a furnace as fine particles the ores can cause capping of the melt which can lead to eruption or violent gas release. Various mineral or carbonaceous particles in dust form are dangerous when inhaled. Mineral fines could be valuable and if in the right format could be subjected to further recovery processes.
It is known to dispose of such small particles or dust as a suspension in water. The suspension might be difficult to dry and could be dangerous if released from slimes dams by flooding. Wet small particles are particularly difficult to handle or recover. When the particles are subjected to further heat processing it is important to note that the feedstock in a sinter plant cannot contain too high a proportion of fine particles because the fine particles interfere with the porosity or permeability of the sinter bed and impede the movement of gas.
In the size upgrading of small particles, the most frequently used method has been by compression. Briquetting is an example and this can be achieved in a variety of ways including by way of contrary rotating roll presses or by way of a revolving, outer perforated cage wherein a roll press forces material caught between the roll press and the outer cage through apertures in the outer cage. Binders such as starch are optionally used in these processes. Alternatively a synergistic inclusion can be used, such as biomass in coal or Bentonite in metal ores.
The compression of small particles requires substantial energy, capital expenditure and specialised equipment which is prone to wear and tear. Additionally the compression of small particles inhibits subsequent drying of the agglomerate and can influence the behaviour of the agglomerate when it is subjected to high temperature. This is a function of the ease or otherwise of the release of either water vapour or volatiles, which can either impede drying or can result in the catastrophic disintegration of the agglomerate when subjected to temperature shock.
Another known method for the size upgrading of small particles is by first mixing the small particles with a liquid to form a damp and relatively free flowing mixture. The mixture is then deposited onto an inclined and rotating disc or cylinder and which increases in size by the aggregation of the particles on the disc. The size of the resulting agglomerate depends on its dwell time, the diameter and speed of the disc and whether or not a binder is sprayed onto the particles with further dry feedstock or not. The result is a rounded pebble which may be sintered. Once again the method is characterized by various advantages and disadvantages.
The shape of an agglomerate is of importance as a round shape minimises attrition on subsequent handling and variable size allows for good packing and better logistical efficiency. A pillow shaped particle or an extruded one has the disadvantage of high attrition associated with sharp edges and the advantage of the absence of cascading from or on an inclined plain such as a conveyor belt.
Preferably the size upgrading of small particles should allow for the control of the upgraded particle size across a wide spectrum. Additionally the size upgrading should take place during a continuous production with low wear, low capital cost, low running expense, ease of control and simplicity. The shape, integrity, resistance to attrition, resistance to water and behaviour at high temperature of the agglomerate should be controllable.